The invention relates to a wire electrode for spark erosion cutting of metals, electrically-conducting ceramics etc., comprising either a homogenous centre made of a metal or of a metal alloy, or a composite centre and comprising at least two sheath coatings made of Zn alloys, with said coatings wearing during erosion.
In the development of wire electrodes for spark erosion cutting of metals or other electrically-conducting bodies it has been proven advantageous if a sheathed electrode is used in which the centre not only provides the necessary strength but also the electrical conductivity, wherein the actual erosion process is carried out by a sheath coating which comprises pure zinc or a zinc alloy. While a pure coating of zinc provides good removal performance, the pure zinc coating wears however so quickly that it is completely used up before the erosion wire emerges from the cutting gap of a moderately high workpiece, so that eventually, the electrode cuts with the core material. This results in the cutting performance of the electrode being altogether considerably reduced.
For this reason it is known (EP 0 312 674) to provide a xcex2-sheath coating which, while in theory, due to the reduced zinc content, is said to have a lesser cutting performance than a pure zinc sheath coating, but which due to its far lower rate of wear in the practical cutting process provides a better cutting performance already at low workpiece heights.
Furthermore, from DE 195 10 740 A1 it is known to produce the sheath coating from a xcex3-brass, which leads to an increased removal performance being achieved because the zinc fraction in the xcex3-brass is higher than it is in the xcex2-brass. Practical operation shows however, that this xcex3-coating wears far more quickly than does a xcex2-sheath coating, so that the advantages are noticeable only in the case of low workpiece heights.
Furthermore, from DE 199 13 694 A1, the manufacture of a wire electrode comprising two sheath coatings, one on top of the other, is known, wherein the lower sheath coating comprises a xcex2-brass while the upper sheath coating comprises pure zinc or a zinc alloy comprising in excess of 80% zinc. This electrode is a so-called combination electrode which is equally suitable for high-speed cutting and for fine cutting where rapid wear of the sheath coating is less important. If such an electrode is used for high-speed cutting, then in this method, the outer sheath coating which comprises zinc and which wears rapidly, may improve the ignition behaviour of the electrode, while the actual erosion process takes place by means of the xcex2-sheath coating. By contrast, if this electrode is used for fine cutting, only the outer sheath coating is consumed while the underlying xcex2-sheath coating remains. This electrode provides an advantage in that it is possible with one and the same wire electrode, i.e. without any set-up time, to carry out high-speed cutting as well as fine cutting where it is important to achieve an extremely smooth surface.
It is the object of the invention to propose an electrode for high-speed cutting which when compared to an electrode comprising a xcex2-sheath coating and when compared to an electrode comprising a xcex3-sheath coating, features a better cutting performance and moreover comparatively less wear.
According to the invention, this object is met in that the inner sheath coating predominantly comprises xcex2-brass, while the outer sheath coating predominantly comprises xcex3-brass, wherein the fraction of xcex2/xcex2xe2x80x2-phase or xcex3-phase in the two sheath coatings is at least 60%, and wherein the ratio of the coating thickness of xcex2-brass to xcex3-brass is between 0.3 and 7, and the sum of the thicknesses of both sheath coatings in relation to the external diameter of the electrode is between 0.1 and 0.3.
Surprisingly, the combination of an underlying xcex2-sheath coating with an overlying xcex3-sheath coating leads to a cutting performance which is better than that which can be achieved with a pure xcex3-sheath coating or with a pure xcex2-sheath coating.